arXiv subjects

Stefano Forte

Publications and source records attributed to Stefano Forte.

At least 163 records · Page 9Linked to original sources

arXiv1998

A proof of the irreversibility of renormalization group flows in four dimensions

We present a proof of the irreversibility of renormalization group flows, i.e. the c-theorem for unitary, renormalizable theories in four (or generally even) dimensions. Using Ward identities for scale transformations and spectral representation arguments, we show that the c-function based on the trace of the energy-momentum tensor (originally suggested by Cardy) decreases monotonically along renormalization group trajectories. At fixed points this c-function is stationary and coincides with the coefficient of the Euler density in the trace anomaly, while away from fixed points its decrease is due to the decoupling of positive--norm massive modes.

hep-th↗

arXiv1998

Are Parton Distributions Positive?

We show that the naive positivity conditions on polarized parton distributions which follow from their probabilistic interpretation in the naive parton model are reproduced in perturbative QCD at the leading log level if the quark and gluon distribution are defined in terms of physical processes. We show how these conditions are modified at the next-to-leading level, and discuss their phenomenological implications, in particular in view of the determination of the polarized gluon distribution

hep-ph↗

arXiv1998

Corrections at Small x

We show that the all order summation of small x logarithms cannot be included in the Q^2 evolution of structure functions because the NLLx corrections overwhelm the LLx contribution.

hep-ph↗

arXiv1997

What Can We Learn From Polarized Structure Function Data?

We summarise the perturbative QCD analysis of the structure function data for g_1 from longitudinally polarized deep inelastic scattering from proton, deuteron and neutron targets, with particular emphasis on testing sum rules, determining helicity fractions, and extracting the strong coupling from both scaling violations and the Bjorken sum rule.

hep-ph↗

arXiv1997

Progress in perturbative QCD

We briefly summarize some recent theoretical developments in perturbative QCD, emphasizing new ideas which have led to widening the domain of applicability of perturbation theory. In particular, it is now possible to calculate efficiently processes with many partons; the high order behavior of perturbation theory can be at least partly understood by going beyond leading twist accuracy; factorization with more than one hard scale (such as in DIS with heavy quarks) can be made consistent with the renormalization group; and large infrared logs can be resummed beyond the renormalization group. The use of the renormalization group to resum large longitudinal scales may allow the use of perturbation theory even in the absence of a large transverse scale.

hep-ph↗

arXiv1997

Asymptotic Freedom at Small x

We describe how perturbative QCD may be applied to inelastic e-p scattering at high center-of-mass energies, i.e. at small x and fixed Q^2.

hep-ph↗

arXiv1997

Determination of the Bjorken Sum and Strong Coupling from Polarized Structure Functions

We present a NLO perturbative analysis of all available data on the polarized structure function g_1(x,Q^2) with the aim of making a quantitative test of the validity of the Bjorken sum rule, of measuring α_s, and of deriving helicity fractions. We take particular care over the small x extrapolation, since it is now known that Regge behaviour is unreliable at perturbative scales. For fixed α_s we find that if all the most recent data are included g_A=1.18\pm0.09, confirming the Bjorken sum rule at the 8% level. We further show that the value of α_s is now reasonably well constrained by scaling violations in the structure function data, despite the fact that it cannot yet be reliably fixed by the value of the Bjorken sum: our final result is α_s(m_Z) = 0.120+0.010-0.008. We also confirm earlier indications of a sizeable positive gluon polarization in the nucleon.

hep-ph↗

arXiv1996

Polarized structure functions: a status report

We review the present status of polarized structure functions measured in deep-inelastic scattering. We discuss the x and Q^2 dependence of the structure function g_1, and how it can be used to test perturbative QCD at next-to-leading order and beyond. We summarize the current knowledge of polarized parton distributions, in particular the determination of the first moment of the quark and gluon distributions and the axial charge of the nucleon. We critically examine what future experiments could teach us on the polarized structure of the nucleon.

hep-ph↗

arXiv1996

Double Asymptotic Scaling '96

We review recent HERA data on the structure function F_2 at small x and large Q^2. We show that the salient features of the data are revealed by comparing them to the double asymptotic scaling behaviour which F_2 is predicted to satisfy in perturbative QCD.

hep-ph↗

arXiv1996

Measurement of the Polarized Structure Function g_1^p at HERA

We present estimates of possible data on spin-dependent asymmetries in inclusive scattering of high energy polarized electrons by high energy polarized protons at HERA, including statistical errors, and discuss systematic uncertainties. We show that these data would shed light on the small x behaviour of the polarized structure function g_1, and would reduce substantially the uncertainty on the determination of the polarized gluon distribution.

hep-ph↗

arXiv1996

Polarized Partons at Next-to-Leading Order

We discuss the determination of polarized parton distributions from a next-to-leading order analysis of recent experimental data. We extract the first moment of the polarized quark and gluon distribution and assess the corresponding uncertainties, with special regard to those related to scheme dependence.

hep-ph↗

arXiv1996

Double scaling violations

We discuss the theoretical implications of the scaling properties of F_2^p at small x observed in recent HERA experiments. We show that low Q^2 data display double scaling violations which are adequately described by NL ln Q^2 corrections. Scaling violations due to summations of leading and subleading ln 1/x beyond NLO in ln Q^2 are however disfavoured by the data.

hep-ph↗

arXiv1996

Inclusive Measurement of the Strong Coupling at HERA

We describe the measurement of the strong coupling alpha_s from data on inclusive DIS at high energies. We present new results using the 1994 data from H1, and confirm directly the expected running of alpha_s.

hep-ph↗

arXiv1995

Universality and Scaling in Perturbative QCD at Small x

We present a pedagogical review of the universal scaling properties displayed by the structure function F_2 at small x and large Q^2 as measured at HERA. We first describe the derivation of the double asymptotic scaling of F_2 from the leading-order Altarelli-Parisi equations of perturbative QCD. Universal next-to-leading order corrections to scaling are also derived. We explain why the universal scaling behaviour is spoiled when the initial distributions rise too steeply by considering the nonsinglet distribution F_2^p-F_2^n as an explicit example. We then examine the stability of double scaling to the inclusion of higher order singularities, explaining how the perturbative expansion at small x can be reorganized in such a way that each order is given by the sum of a convergent series of contributions which are of arbitrarily high order in the coupling. The wave-like nature of perturbative evolution is then shown to persist throughout almost all the small x region, giving rise asymptotically to double scaling for a wide class of boundary conditions.

hep-ph↗

arXiv1995

Polarized Structure Functions: a Theoretical Update

We review recent developments in the theory and phenomenology of polarized structure functions. We summarize recent experimental data on the proton and deuteron structure function $g_1$, and their impact on the understanding of polarized sum rules. Specifically, we discuss how accurate measurements of the singlet and nonsinglet first moment of $g_1$ test perturbative and nonperturbative QCD, and critically examine the way these measurements are arrived at. We then discuss how the extraction of structure functions from the data can be improved by means of a theoretical analysis of their $x$ and $Q^2$ dependence, and how, conversely, experimental information on this dependence can be used to pin down the polarized parton content of the nucleon.

hep-ph↗

arXiv1994

Polarized Structure Functions: Theory and Phenomenology

We review the theory of polarized structure functions measured in deep-inelastic lepton-nucleon scattering, focusing on the most recent developments. We concentrate on the structure function $g_1$, emphasizing the phenomenological problems related to the extraction of the proton and neutron $g_1(x,Q^2)$ from the data, and to the determination of its moments, especially the first moment. In particular, we discuss the theoretical and experimental uncertainties due to the $Q^2$ dependence of the data, small and large $x$ extrapolations, QCD loop corrections, higher twist corrections, and, in connection to neutron experiments, nuclear effects. We also discuss the current status of the sum rules satisfied by the first moment of $g_1$ and their theoretical interpretation.

hep-ph↗

arXiv1994

Anomalous Evolution of Nonsinglet Structure Functions

We review a formalism that includes the effects of nonperturbative U(1) symmetry breaking on the QCD evolution of nonsinglet structure functions. We show that a strong scale dependence is generated in an intermediate energy range $0.5 \lsim Q^2 \lsim 5$~GeV$^2$ for all values of $x$. We show that this explains naturally the observed violation of the Gottfried sum, and allows a determination of the shape of the nonsinglet structure function, in excellent agreement with experiment. We argue that these effects may affect the determination of $α_s$ from deep--inelastic scattering.

hep-ph↗

arXiv1993

An Instanton-Induced Contribution to the Decay of the Eta_c into P-Pbar

We compute the decay rate for the process $η_c\to p\bar p$ using an effective helicity-flipping proton-antiproton-gluon vertex which incorporates nonperturbative chiral symmetry breaking effects induced by instantons. We fix the strength of the vertex by requiring it to account for the screening of the proton's axial charge observed in deep-inelastic scattering, and we estimate the size of the instanton effects by assuming them to depend linearly on the instanton density. We find that, despite a large suppression, the instanton-induced process occurs with a sizable rate comparable to the observed one, whereas the process is forbidden in perturbative QCD and not understood using standard methods.

hep-ph↗